The pulse width modulation (PWM) is a very effective technique for controlling an analog circuit utilizing digital output of a microprocessor, and is widely applied in many different fields from measuring, communication, to power control and conversion.
Please refer to FIGS. 1, 2 and 3A, which shows a conventional PWM circuit applied to a fan motor (not shown). The PWM circuit includes a direct current (DC) power supply unit 110, a voltage-dividing resistor unit 120, a comparing unit 130, and a signal generating unit 140.
The DC power supply unit 110 is a variable DC voltage source Vin of 2 to 5 volts, for example, for supplying a voltage source to the voltage-dividing resistor unit 120.
The voltage-dividing resistor unit 120 includes a first resistor element R7 and a second resistor element R8, and a first node voltage Vr coupled to and between the first and the second resistor element R7 and R8. The voltage-dividing resistor unit 120 has a voltage value that varies with the voltage value supplied by the DC power supply unit 110.
The signal generating unit 140 may be a triangular-wave generator 141 for generating a triangular-wave signal. The triangular-wave signal has fixed high level VH and low level LV, which do not vary with changes in the voltage value of the DC power supply unit 110.
The comparing unit 130 may be a comparator 131 for receiving and comparing the signals from the signal generating 140 and the voltage-dividing resistor unit 120, and outputting a pulse width modulated (PWM) signal.
When the variable DC power source Vin supplies a voltage source of 2 to 5 volts, for example, to the voltage-dividing resistor unit 120, the latter calculates using a formula based on the voltage-divider theorem to derive the voltage signal to be sent to the comparing unit 130. Meanwhile, when the triangular-wave signal generated by the signal generating unit 140 is also sent to the comparing unit 130, the comparing unit 130 compares the voltage signal and the triangular-wave signal and outputs a PWM signal to drive the fan motor (not shown) to operate. FIG. 2 is a voltage-speed curve for the conventional PWM circuit of FIG. 1. As can be clearly seen from FIG. 2, the voltage-speed curve T1 is a non-smooth curve including a breaking point P.
Please refer to FIGS. 3A, 3B, and 3C, which are applied circuit diagram, waveform graph, and duty cycle-voltage curve for an embodiment of the conventional PWM circuit of FIG. 1. It is already known that the high level HV and the low level LV of the triangular-wave generator 141 is set to 2V and 1V, respectively, and the first resistor element R7 is set to 10KΩ. Given that the duty cycle of the PWM signal is 50% and the variable DC voltage source Vin is 2V; then, the first node voltage Vr may be calculated using the following formula:
            50      ⁢      %        =                            VH          -          Vr                          VH          -          VL                    =                                                  2              -              Vr                                      2              -              1                                ⇒          Vr                =                  1.5          ⁢                                          ⁢          V                          Vr    =                            Vin          ×                                    R              ⁢                                                          ⁢              8                                      (                                                R                  ⁢                                                                          ⁢                  7                                +                                  R                  ⁢                                                                          ⁢                  8                                            )                                      ⇒        1.5            =                                    2            ×                                          R                ⁢                                                                  ⁢                8                                            (                                                      10                    ⁢                                                                                  ⁢                    K                                    +                                      R                    ⁢                                                                                  ⁢                    8                                                  )                                              ⇒                      R            ⁢                                                  ⁢            8                          =                  30          ⁢                                          ⁢          K          ⁢                                          ⁢          Ω                    From the above calculations, it is derived that the first node voltage Vr is 1.5V, and the second resistor element R8 is 30KΩ.
Alternatively, given that the duty cycle of the PWM signal is 100%, and the variable DC voltage source Vin is 5V; then, the first node voltage Vr may be derived using the following formula:
  Vr  =                    Vin        ×                              R            ⁢                                                  ⁢            8                                (                                          R                ⁢                                                                  ⁢                7                            +                              R                ⁢                                                                  ⁢                8                                      )                              ⇒      Vr        =                            5          ×                                    30              ⁢                                                          ⁢              K                                                      10                ⁢                                                                  ⁢                K                            +                              30                ⁢                                                                  ⁢                K                                                    ⇒        Vr            =              3.75        ⁢                                  ⁢        V            Therefore, when the variable DC voltage source Vin is 5V, the first node voltage Vr is 3.75V. On the other hand, given that when the first node voltage is 2V to just reach the high level VH, then the variable DC voltage source Vin may be calculated using the following formula:
  Vr  =                    Vin        ×                              R            ⁢                                                  ⁢            8                                (                                          R                ⁢                                                                  ⁢                7                            +                              R                ⁢                                                                  ⁢                8                                      )                              ⇒      2        =                            Vin          ×                                    30              ⁢                                                          ⁢              K                                      (                                                10                  ⁢                                                                          ⁢                  K                                +                                  30                  ⁢                                                                          ⁢                  K                                            )                                      ⇒        Vin            =              2.67        ⁢                                  ⁢        V            FIG. 3C is a duty cycle-voltage curve plotted according to the values obtained from the above calculations. As can be seen from FIG. 3C, the duty cycle-voltage curve D1 starts extending upward at a point corresponding to the duty cycle of 50% and the variable DC voltage source Vin of 2V until the curve reaches the point corresponding to the variable DC voltage source Vin of 2.67V. Then, the duty cycle-voltage curve D1 stably extends horizontally until it reaches the point corresponding to the variable DC voltage source Vin of 5V. As can be clearly seen from FIG. 3C, the duty cycle-voltage curve D1 is a non-smooth curve.
Therefore, the conventional PWM circuit has the following disadvantages: (1) it has a non-smooth duty cycle-voltage curve, which makes the conventional PWM circuit unstable; and (2) it has a non-smooth voltage-speed curve, which makes the conventional PWM circuit unstable.
It is therefore tried by the inventor to develop an improved PWM structure to overcome the problems existed in the conventional PWM circuit.